1. Field of the Invention
The present invention relates to satellite navigation systems, such as the Global Positioning System (GPS), and, in particular, to satellite navigation receivers and associated methods of controlling satellite navigation receivers.
2. Description of the Related Art
Satellite navigation systems have been available to the public for many years. In particular, the Global Positioning System (GPS), which was developed by the U.S. Department of Defense, was made available for public use in 1983. Other satellite navigation systems are currently in development, including the GLONASS system (Russia), the Galileo positioning system (Europe), the COMPASS navigation system (China), and IRNSS (India).
The GPS system includes 24 Medium Earth Orbit (MEO) satellites that transmit precise timing signals to terrestrial receivers. Unlike geostationary orbiting satellites, MEO satellites move across the sky relative to the position of a terrestrial receiver. Thus, a given MEO satellite may be visible, or not visible, to a terrestrial receiver at any moment. However, the MEO satellites are arranged in six orbital planes with four satellites each, so that a number of the satellites are visible from any point on earth at any moment. By examining the signals received from a plurality of the satellites, a GPS receiver can determine its position.
A GPS receiver can calculate its position using the GPS signals from four or more GPS satellites. The signals from the four satellites are used to solve for four variables: x-, y-, z-position, and time. The GPS receiver can then convert these values into latitude/longitude information, and can typically display the location of the receiver on a map.
The GPS signal includes a Navigation Message that provides the time-of-day, GPS week number and satellite health information. The Navigation Message also includes ephemeris data and an almanac. The ephemeris data provides detailed information about the orbit of the satellite that transmitted the navigation message. The almanac provides coarse orbit and status information for each satellite in the constellation, an ionospheric model, and information to relate GPS derived time to Coordinated Universal Time (UTC).
The Navigation Messages are sent in frames at 50 bits/second, each taking 30 seconds to transmit 1500 bits. The time needed to acquire the ephemeris is significant, as the ephemeris data requires at least 30 seconds to be received completely.
Because satellite signals are relatively weak, they cannot readily penetrate into buildings, shelters, tunnels, or other covered locations. When a GPS receiver is taken into such a location, all of the GPS signals then being processed by the GPS receiver can become so heavily attenuated that they are lost entirely. The GPS receiver may then begin a process of signal acquisition in an attempt to locate a sufficient number of GPS signals to allow the receiver to determine its geographic location. Given the amount of time required to search for the GPS signals, and the number of signals that a GPS receiver must search for, a GPS receiver can consume significant amounts of power when all of the GPS signals are lost.